Fish Adaptations

The body plan of a fish makes it well suited to live in water. A streamlined shape and a muscular tail enable most fishes to move rapidly through the water. Paired fins allow fishes to maneuver right or left, up or down, and backward or forward. Unpaired fins on the back and belly increase stability. In addition, most fishes secrete a mucus that reduces friction as they swim, and that helps protect them from infections.

Most of the tissues in a fish's body are denser than water. By controlling the amount of gas in their bodies, many fishes can regulate their vertical position in the water. Some fishes also store lipids, which are less dense than water and therefore add buoyancy.

Fishes need to absorb oxygen and rid themselves of carbon dioxide. However, scales on fishes limit diffusion through the skin. Instead, most exchanges between water and blood take place across the membranes of gills—the internal respiratory organs of fishes.

Homeostasis

The concentration of solutes in a fish's body usually differs from the concentration of solutes in the water in which the fish swims. The body of a freshwater fish is hypertonic. It has a higher concentration of solutes than the surrounding water does, so the fish tends to gain water and lose ions, such as sodium and chloride ions, through diffusion. Most saltwater fishes are hypotonic; they contain lower concentrations of solutes than their surroundings do. Thus, saltwater fishes tend to lose water and gain ions.

Like all organisms, fishes must also rid themselves of the waste products produced by metabolism. The kidneys and gills play important roles in maintaining homeostasis in the tissues and in getting rid of metabolic wastes. The kidneys filter the blood and help regulate the concentration of ions in the body. The gills release wastes, such as carbon dioxide and ammonia, and either absorb or release ions, depending on whether the fish lives in fresh water or in salt water.

objectives

• Identify three characteristics that make fishes well suited to aquatic life.

• Describe three sensory systems in fishes.

• Evaluate the similarities between jawless fishes and early vertebrates.

• Identify two characteristics of cartilaginous fishes.

• Contrast reproduction in lampreys with reproduction in cartilaginous fishes.

Hagfish Depletion

Hagfishes are economically important. Most "eelskin" products, such as wallets, are actually made from the tanned skin of hagfishes. The demand for these products is so high that hagfish populations in some parts of the world have been almost wiped out by overfishing.

Sensory Functions

Fishes have a variety of organs that allow them to sense the world around them. Fish can sense light, chemicals, and sound. Some can also sense electrical and magnetic fields. Fish eyes are similar to eyes of land vertebrates. Many bony fishes have color vision, but most cartilaginous fishes do not.

Chemoreception is the ability to detect chemicals in the environment. Chemoreception includes the senses of smell and taste. Most fish have one or two nostrils, which lead to sensory cells located in olfactory sacs on both sides of the head. Sharks and salmon are examples of fish with a well-developed sense of smell. Fishes have taste buds located in their mouths. They may also have taste buds on their lips, fins, and skin and on whisker-like organs near their mouths called barbels.

Fishes have a unique organ called the lateral line, which allows them to sense vibration in the water. The lateral line is made of a system of small canals in the skin. The canals are lined with cells that are sensitive to vibration. Fishes also perceive sound waves with their inner ears, which contain a fluid-filled set of canals that contain sensory cells similar to those in the lateral line canals.

Cartilaginous fishes also have sense organs called the ampullae of Lorenzini that can detect weak electrical fields, such as those given off by muscles when they contract. This system has been shown to help them locate prey.

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